Network Working Group J. Borkenhagen
Internet-Draft AT&T
Updates: 1997 (if approved) R. Bush
Intended status: Standards Track IIJ & Arrcus
Expires: December 12, 2019 R. Bonica
Juniper Networks
S. Bayraktar
Cisco Systems
June 10, 2019
Well-Known Community Policy Behaviordraft-ietf-grow-wkc-behavior-07
Abstract
Well-Known BGP Communities are manipulated differently across various
current implementations; resulting in difficulties for operators.
Network operators should deploy consistent community handling across
their networks while taking the inconsistent behaviors from the
various BGP implementations into consideration.. This document
recommends specific actions to limit future inconsistency, namely BGP
implementors must not create further inconsistencies from this point
forward.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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Internet-Draft Well-Known Community Policy Behavior June 2019
This document recommends specific actions to limit future
inconsistency, namely BGP implementors MUST NOT create further
inconsistencies from this point forward.
2. Manipulation of Communities by Policy
[RFC1997] says:
"A BGP speaker receiving a route with the COMMUNITIES path attribute
may modify this attribute according to the local policy."
One basic operational need is to add or remove one or more
communities to the set. The focus of this document is another common
operational need, to replace all communities with a new set. To
simplify this second case, most BGP policy implementations provide
syntax to "set" community that operators use to mean "remove any/all
communities present on the route, and apply this set of communities
instead."
Some operators prefer to write explicit policy to delete unwanted
communities rather than using "set;" i.e. using a "delete community
*:*" and then "add community x:y ..." configuration statements in an
attempt to replace all communities. The same community manipulation
policy differences described in the following section exist in both
"set" and "delete community *:*" syntax. For simplicity, the
remainder of this document refers only to the "set" behaviors, which
we refer to collectively as each implementation's '"set" directive.'
3. Community Manipulation Policy Differences
Vendor implementations differ in the treatment of certain Well-Known
communities when modified using the syntax to "set" the community.
Some replace all communities including the Well-Known ones with the
new set, while others replace all non-Well-Known Communities but do
not modify any Well-Known Communities that are present.
These differences result in what would appear to be identical policy
configurations having very different results on different platforms.
4. Documentation of Vendor Implementations
In this section we document the syntax and observed behavior of the
"set" directive in several popular BGP implementations to illustrate
the severity of the problem operators face.
In Juniper Networks' Junos OS, "community set" removes all
communities, Well-Known or otherwise.
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Internet-Draft Well-Known Community Policy Behavior June 2019
In Cisco IOS XR, "set community" removes all communities except for
the following:
+-------------+-----------------------------------+
| Numeric | Common Name |
+-------------+-----------------------------------+
| 0:0 | internet |
| 65535:0 | graceful-shutdown |
| 65535:1 | accept-own rfc7611 |
| 65535:65281 | NO_EXPORT |
| 65535:65282 | NO_ADVERTISE |
| 65535:65283 | NO_EXPORT_SUBCONFED (or local-AS) |
+-------------+-----------------------------------+
Communities not removed by Cisco IOS XR
Table 1
Cisco IOS XR does allow Well-Known communities to be removed only by
explicitly enumerating one at a time, not in the aggregate; for
example, "delete community accept-own". Operators are advised to
consult Cisco IOS XR documentation and/or Cisco support for full
details.
On Extreme networks' Brocade NetIron: "set community X" removes all
communities and sets X.
In Huawei's VRP product, "community set" removes all communities,
Well-Known or otherwise.
In OpenBGPD, "set community" does not remove any communities, Well-
Known or otherwise.
Nokia's SR OS has several directives that operate on communities.
Its "set" directive is called using the "replace" keyword, replacing
all communities, Well-Known or otherwise, with the specified
communities.
4.1. Note on an Inconsistency
The IANA publishes a list of Well-Known Communities [IANA-WKC].
Cisco IOS XR's set of Well-Known communities that "set community"
will not overwrite diverges from the IANA's list of Well-Known
communities. Quite a few Well-Known communities from IANA's list do
not receive special treatment in Cisco IOS XR, and at least one
community on Cisco IOS XR's special treatment list, internet == 0:0,
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Internet-Draft Well-Known Community Policy Behavior June 2019
is not formally a Well-Known Community as it is not in [IANA-WKC];
but taken from the Reserved range [0x00000000-0x0000FFFF].
This merely notes an inconsistency. It is not a plea to 'protect'
the entire IANA list from "set community."
5. Note for Those Writing RFCs for New Community-Like Attributes
When establishing new [RFC1997]-like attributes (large communities,
wide communities, etc.), RFC authors should state explicitly how the
> new attribute is to be handled.
6. Action Items
Network operators are encouraged to limit their use of the "set"
directive (within reason), to improve consistency across platforms.
Unfortunately, it would be operationally disruptive for vendors to
change their current implementations.
Vendors SHOULD clearly document the behavior of "set" directive in
their implementations.
Vendors MUST ensure that their implementations' "set" directive
treatment of any specific community does not change if/when that
community becomes a new Well-Known Community through future
standardization. For most implementations, this means that the "set"
directive MUST continue to remove the community; for those
implementations where the "set" directive removes no communities,
that behavior MUST continue.
Given the implementation inconsistencies described in this document,
network operators are urged never to rely on any implicit
understanding of a neighbor ASN's BGP community handling. I.e.,
before announcing prefixes with NO_EXPORT or any other community to a
neighbor ASN, the operator should confirm with that neighbor how the
community will be treated.
7. Security Considerations
Surprising defaults and/or undocumented behaviors are not good for
security. This document attempts to remedy that.
8. IANA Considerations
The IANA is requested to list this document as an additional
reference for the [IANA-WKC] registry.
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